Chang Yi-Hsuan, Yan Hao-Zhi, Liou Ruey-Fen
Department of Plant Pathology and Microbiology, National Taiwan University, Taipei, 106, Taiwan.
Mol Plant Pathol. 2015 Feb;16(2):123-36. doi: 10.1111/mpp.12166. Epub 2014 Aug 8.
The interaction between Phytophthora pathogens and host plants involves the exchange of complex molecular signals from both sides. Recent studies of Phytophthora have led to the identification of various apoplastic elicitors known to trigger plant immunity. Here, we provide evidence that the protein encoded by OPEL of Phytophthora parasitica is a novel elicitor. Homologues of OPEL were identified only in oomycetes, but not in fungi and other organisms. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) revealed that OPEL is expressed throughout the development of P. parasitica and is especially highly induced after plant infection. Infiltration of OPEL recombinant protein from Escherichia coli into leaves of Nicotiana tabacum (cv. Samsun NN) resulted in cell death, callose deposition, the production of reactive oxygen species and induced expression of pathogen-associated molecular pattern (PAMP)-triggered immunity markers and salicylic acid-responsive defence genes. Moreover, the infiltration conferred systemic resistance against a broad spectrum of pathogens, including Tobacco mosaic virus, the bacteria wilt pathogen Ralstonia solanacearum and P. parasitica. In addition to the signal peptide, OPEL contains three conserved domains: a thaumatin-like domain, a glycine-rich protein domain and a glycosyl hydrolase (GH) domain. Intriguingly, mutation of a putative laminarinase active site motif in the predicted GH domain abolished its elicitor activity, which suggests enzymatic activity of OPEL in triggering the defence response.
疫霉病原体与寄主植物之间的相互作用涉及双方复杂分子信号的交换。最近对疫霉的研究已鉴定出多种已知能触发植物免疫的质外体激发子。在此，我们提供证据表明寄生疫霉的OPEL编码的蛋白质是一种新型激发子。仅在卵菌纲中鉴定出了OPEL的同源物，而在真菌和其他生物体中未发现。定量逆转录-聚合酶链反应（RT-PCR）显示，OPEL在寄生疫霉的整个发育过程中均有表达，在植物感染后尤其高度诱导表达。将来自大肠杆菌的OPEL重组蛋白渗入烟草（品种Samsun NN）叶片中，导致细胞死亡、胼胝质沉积、活性氧的产生，并诱导病原体相关分子模式（PAMP）触发的免疫标记物和水杨酸响应防御基因的表达。此外，这种渗入赋予了对多种病原体的系统抗性，包括烟草花叶病毒、青枯病病原菌茄科雷尔氏菌和寄生疫霉。除信号肽外，OPEL包含三个保守结构域：一个类thaumatin结构域、一个富含甘氨酸的蛋白质结构域和一个糖基水解酶（GH）结构域。有趣的是，预测的GH结构域中一个假定的海带多糖酶活性位点基序的突变消除了其激发子活性，这表明OPEL在触发防御反应中具有酶活性。
